Plasma reactor for the treatment of large size substrates

ABSTRACT

A radiofrequency plasma reactor (1) for the treatment of substantially large sized substrates is disclosed, comprising between the electrodes (3, 5) of the plasma reactor a solid or gaseous dielectric layer (11) having a non planar-shaped surface-profile, said profile being defined for compensating a process non uniformity in the reactor or generating a given distribution profile.

[0001] The invention relates to a capacitively coupled radiofrequency(RF) plasma reactor and to a process for treating at least one substratein such a reactor. Especially, the present invention applies to a largesize capacitive capacitively coupled (RF) plasma reactor.

[0002] Often, such a reactor is known as a “capacitive” RF glowdischarge reactor, or planar plasma capacitor or parallel plate RFplasma reactor, or as a combination of the above named.

[0003] Capacitive RF plasma reactors are typically used for exposing asubstrate to the processing action of a glow discharge. Variousprocesses are used to modify the nature of the substrate surface.Depending on the process and in particular the nature of the gasinjected in the glow discharge, the substrate properties can be modified(adhesion, wetting), a thin film added (chemical vapour deposition CVD,diode sputtering) or another thin film selectively removed (dryetching).

[0004] The table shown below gives a simplified summary of the variousprocesses possibly performed in a low pressure capacitive discharge.Industry Substrate type Process Inlet gas nature Semi- wafer SurfaceCleaning Ar conduc- up to 30 cm PECVD SiH₄, . . . tor diameter DryEtching CF₄, SF₆, Cl₂, . . . Ashing O₂, Disks Polymer or glass Diodesputtering Ar + others for up to 30 cm PECVD Organometallics memorydiameter Surface activation O₂, etc . . . Flat Glass Same as for Same asfor display up to 1.4 m diagonal semiconductors semiconductors Win-Glass up to 3 m Cleaning/ Air, Argon - dow width, foil, plasticactivation, Monomer, Nitrogen, pane or metal Nitriding, polymer . . .Web PECVD coaters

[0005] The standard frequency of the radiofrequency generators mostlyused in the industry is 13.56 MHz. Such a frequency is allowed forindustrial use by international telecommunication regulations. However,lower and higher frequencies were discussed from the pioneering days ofplasma capacitor applications. Nowadays, for example for PECVDapplications, (plasma enhanced chemical vapour deposition) there is atrend to shift the RF frequency to values higher than 13.56 MHz, thefavourite values being 27.12 MHz and 40.68 MHz harmonics of 13.56 MHz).

[0006] So, this invention applies to RF frequencies (1 to 100 MHzrange), but it is mostly relevant to the case of higher frequencies(above 10 MHz). The invention can even be applied up to the microwaverange (several GHz).

[0007] An important problem was noted especially if the RF frequency ishigher than 13.56 MHz and a large size (surface) substrate is used, insuch a way that the reactor size is no more negligible relative to thefree space wave length of the RF electromagnetic wave. Then, the plasmaintensity along the reactor can no longer be uniform. Physically, theorigin of such a limitation should lie in the fact that the RF wave isdistributed according to the beginning of a “standing wave” spacialoscillation within the reactor. Other non uniformities can also occur ina reactor, for example non uniformities induced by the reactive gasprovided for the plasma process.

[0008] It is an object of the invention to propose a solution foreliminating, or at least notably reducing, an electromagnetic (or aprocess) non uniformity, in a reactor. Thus, according to an importantfeature of the invention, an improved capacitively coupled RF plasmareactor should comprise:

[0009] at least two electrically conductive electrodes spaced from eachother, each electrode having an external surface,

[0010] an internal process space enclosed between the electrodes,

[0011] gas providing means for providing the internal process space witha reactive gas,

[0012] at least one radiofrequency generator connected to at least oneof the electrodes, at a connection location, for generating a plasmadischarge in the process space, and potentially an aditional RFgenerator for increasing the ion bombardment on the substrate,

[0013] means for evacuating the reactive gas from the reactor, so thatsaid gas circulates within the reactor, at least in the process spacethereof,

[0014] at least one substrate defining one limit of the internal processspace, to be exposed to the processing action of the plasma discharge,said at least one substrate extending along a general surface and beingarranged between the electrodes,

[0015] characterized in that it further comprises at least onedielectric “corrective” layer extending outside the internal processspace, as a capacitor electrically in series with said at least onesubstrate and the plasma, said at least one dielectric layer havingcapacitance per unit surface values which is not uniform along at leastone direction of said general surface, for compensating a process nonuniformity in the reactor or to generate a given distribution profile.

[0016] In other words, the proposed treating process in the reactor ofthe invention comprises the steps of

[0017] locating the at least one substrate between at least twoelectrodes, the substrate extending along a general surface,

[0018] having a reactive gas (or gas mixture) in an internal processspace arranged between the electrodes,

[0019] having a radiofrequency generator connected to at least one ofthe electrodes, at a connection location,

[0020] having a plasma discharge in at least a zone of the internalprocess space facing the substrate, in such a way that said substrate isexposed to the processing action of the plasma discharge,

[0021] creating an extra-capacitor electrically in series with thesubstrate and the plasma, said extra-capacitor having a profile, and

[0022] defining the profile of the extra-capacitor in such a way that ithas location dependent capacitance per unit surface values along atleast one direction of the general surface of the substrate.

[0023] It is to be noted that such a solution is general. It is validfor all plasma processes, but only for a determined RP frequency.

[0024] The “tailored extra-capacitor” corresponding to theabove-mentioned said (substantially) “dielectric layer” acts as acomponent of a capacitive divider.

[0025] Advantageously, the capacitive variations will be obtainedthrough a non uniform thickness of the layer. Thus, the extra-capacitorwill have a profile having a non planar-shape along a surface.

[0026] For compensating a non uniform voltage distribution across theprocess space of the reactor, said thickness will preferably be definedin such a way that:

[0027] the so-called “corrective layer” is the thickest in front of thelocation in the process space (where the plasma is generated) which isthe farest away from the connection location where the radiofrequencygenerator is connected to said at least one electrode, the distancebeing measured by following the electrode external surface,

[0028] and said thickness preferably decreases from said process spacelocation, as the distance between the process space location and theconnection location on the corresponding electrode decreases.

[0029] Of course, it is to be understood that the above-mentioned“distance” is the shortest of all possible ways.

[0030] So, if the electromagnetic travelling waves induced in theprocess space combine each other near the center of the reactor to forma standing wave having a maximum of voltage in the vicinity of thereactor center, the thickness of the so-called “corrective layer” willbe larger in the vicinity of the center thereof, than at its periphery.

[0031] One solution in the invention for tailoring said “correctivelayer” is to shape at least one surface of the layer in such a way thatthe layer has a non planar-shaped external surface, preferably a curvedconcave surface facing the internal process space where the plasma isgenerated. Various ways can be followed for obtaining such a “non planarshaped” surface on the layer.

[0032] It is a priviledged way in the invention to shape at least one ofthe electrodes, in such a way that said electrode has a nonplanar-shaped surface facing the substrate, and especially a generallycurved concave surface.

[0033] It is another object of the invention to define the compositionor constitution of the so-called “corrective layer”.

[0034] According to a preferred solution, said layer comprises at leastone of a solid dielectric layer and gaseous dielectric layer.

[0035] If the layer comprises such a gaseous dielectric layer, it willpreferably be in gaseous communication with the internal process spacewhere the plasma is generated.

[0036] A substrate comprising a plate having a non planar-shapedexternal surface is also a solution for providing the reactor of theinvention with the so-called “corrective layer”.

[0037] Another object of the invention is to define the arrangement ofthe substrate within the reactor. Therefore, the substrate couldcomprise (or consist in) a solid member arranged against spacing memberslocated between said solid member and one of the electrodes, the spacingmember extending in said “corrective layer” along a main direction andhaving, each, an elongation along said main direction, the elongationsbeing non uniform along the solid member.

[0038] A difficulty induced by such spacing members relates to a localperturbation relative to the-contact between the solid member and thesubstrate.

[0039] So, the invention suggests that the spacing members preferablycomprise a solid end adapted to be arranged against the solid member,said solid end having a space therearound.

[0040] Below, the description only refers to a capacitively coupled RFplasma reactor in which the improvements of the invention notably reducethe electromagnetic non uniformity during the plasma process.

[0041] First of all, for most processing plasmas, the electromagneticpropagation brings really a limitation in RF plasma processing forsubstrate sizes of the order, or larger than 0.5 m² and especiallylarger than 1 m², while the frequency of the RF source is higher than 10MHz. More specifically, what is to be considered is the largestdimension of the substrate exposed to the plasma. If the substrate has asubstantially square surface, said “largest dimension” is the diagonalof the square. So, any “largest dimension” higher than substantially 0.7m is critical.

[0042] A basic problem, which is solved according to the presentinvention, is that, due to the propagative aspect of the electromagneticwave created in the plasma capacitor, the RF voltage across the processspace is not uniform. If a RF source is centrally connected to anelectrode, the RF voltage decreases slightly from the center to theedges of said electrode.

[0043] As above-mentioned, one way to recover a (substantially) uniformRF voltage across the plasma itself, is the following:

[0044] a capacitor is introduced between the electrodes, said capacitorbeing in series with the plasma (and the substrate) in the reactor,

[0045] this extra-capacitor acts with the plasma capacitor itself as avoltage divider tailoring the local RF power distribution, to(substantially) compensate a non uniformity of the process due, forexample, to gas compositional non uniformity, to edge effects or totemperature gradient.

[0046] Below is a more detailed description of various preferredembodiments according to the invention, in reference to drawings inwhich:

[0047]FIGS. 1 and 2 are two schematic illustrations of an improvedreactor according to the invention (FIG. 1 is a section of FIG. 2 alonglines I-I),

[0048]FIGS. 3, 4, 5, 6, 7 and 8 show alternative embodiments of theinternal configuration of such a reactor.

[0049]FIGS. 9, 10, 11, 12 and 13 show further schematic embodiments oftypical processes corresponding to the invention.

[0050]FIG. 14 illustrates the “tailoring” concept applied to a variationof thickness.

[0051] In FIGS. 1 and 2, the reactor is referenced 1. Reactor 1 enclosestwo metallic electrodes 3, 5 which have an outer surface, 3 a, 5 a,respectively. The electrodes are spaced from each other.

[0052] A gas source 7 provides the reactor with a reactive gas (or a gasmixture) in which the plasma is generated through a radiofrequencydischarge (see the above table). Pumping means 8 are further pumping thegas, at another end of the reactor.

[0053] The radiofrequency discharge is generated by a radiofrequencysource 9 connected at a location 9 a to the upper electrode 3. Thelocation 9 a is centrally arranged on the back of the external surface 3a of the electrode.

[0054] These schematic illustrations further show an extra-capacitor 11electrically in series with the plasma 13 and a substrate 15 locatedthereon.

[0055] The plasma 13 can be observed in the internal space (having thesame numeral reference) which extends between the electrode 3 and thesubstrate 15.

[0056] The substrate 15 can be a dielectric plate of a uniform thicknesse which defines the lower limit of the internal process space 13, sothat the substrate 15 is exposed to the processing action of the plasmadischarge. The substrate 15 extends along a general surface 15 a and itsthickness e is perpendicular to said surface.

[0057] The extra-capacitor 11 interposed between the substrate 15 andthe lower electrode 5 induces a voltage modification in such a way thatthe RF voltage (V_(P)) across the plasma (for example along line 17,between the electrode 3 and the substrate 15), is only a fraction of theradiofrequency voltage (V_(RF)) between the electrodes 3, 5.

[0058] It is to be noted that the extra-capacitor 11 is materiallydefined as a dielectric layer (for example a ceramic plate) having a nonuniform thickness e₁ along a direction perpendicular to theabove-mentioned surface 15 a.

[0059] Since the location of the RF source on the electrode 3 iscentral, and because of the arrangement (as illustrated in FIGS. 1 and2) of the above-mentioned elements disposed in the reactor, thethickness e₁ of the dielectric plate 11 is maximal at the center thereofand progressively decreases from said center to its periphery, in such away to compensate the electromagnetic non uniformity in the processspace 13. So, the presence of said relatively thick series capacitor 11reduces the effective voltage across the plasma. Hence, for thecompensation of electromagnetic effects in a large surface reactor asillustrated in FIGS. 1 and 2, the series capacitor 11 has to be a bitthicker in the center of the reactor and must be thinned down toward theperiphery thereof.

[0060] The schematic illustrations of FIGS. 3 to 8 show various possibleconfigurations allowing such a compensation of non uniformity in acapacitively coupled radiofrequency plasma reactor, of the typeillustrated in the above FIGS. 1 and 2. It will be noted thatcombinations of the basic options illustrated in FIGS. 3 to 8 arepossible.

[0061] In FIG. 3, a flat, planar ceramic plate 21 of a uniform thicknesse₂ is attached to the upper electrode 23. There is a tailored spacing 31between the metal electrode 23 and the ceramic plate 21. Above the otherelectrode 25 is arranged a substrate 35 which can be either dielectricor metallic (or electrically conductive on at least one of its surface).

[0062] In FIGS. 3 to 8, the location of the connection between the powersource (such as the RF source 9 of FIGS. 1 and 2) and the correspondingmetallic electrode is supposed to be centrally arranged on saidelectrode, and the general geometry of the reactor is also supposed tobe as illustrated, so that, in such conditions, the tailored layer 31has a back surface 31 a which is curved with a concave regular profilefacing the process space 13.

[0063] Thus, the corresponding upper electrode 23 (the internal limit ofwhich, facing the process space 13, is defined by surface 31 a) has avariable thickness e₃. The dimension e₃ is the thinnest at the center ofthe electrode and the thickest at its periphery.

[0064] The second opposed electrode 25 is generally parallel to thefirst electrode 23 and has a uniform thickness e₄.

[0065] It will be noted that the connection between the solid dielectricplate 21 and the tailored gap 31 is not a gas-tight connection. So, thereactive gas introduced within the process space 13 can circulate in thegap 31 which will preferably have a thickness adapted for avoiding aplasma discharge therein. Providing the “corrective gap” 31 withcomplementary means for avoiding said plasma discharge therein is alsopossible.

[0066] In FIG. 4, the electrode 23 has the same internal profile 31 a asin FIG. 3.

[0067] But, the “corrective layer” is presently a ceramic plate 41having a variable thickness e₅.

[0068] In FIGS. 5 to 8, the substrates 35′ are dielectric substrates.

[0069] In FIG. 5, the above electrode 33 is a planar metallic electrodehaving a uniform thickness e₄. The lower electrode 45 corresponds to theupper electrode 23 of FIG. 3. The electrode 45 has an internal uppersurface 51 b which defines a rear limit for the curved concave gaseous“corrective layer”. Above said layer 51 is arranged a dielectric planarhorizontal plate 21. The ceramic plate 21 of a uniform thickness e₂ isconnected at its periphery to the lower electrode 45 (counterelectrode).The substrate 35′ is arranged on the ceramic plate 21.

[0070] Since the pressure of the reactive gas adapted to be introducedwithin the reactive space is typically between 10⁻¹ Pa to 10³ Pa, thepressure within the gaseous corrective gap can be substantially equal tosaid injected gas pressure. Typically, the reactive gas pressure withinthe plasma discharge zone 13 will be comprised between 1 Pa and 30 Pafor an etching process, and will be comprised between 30 Pa and 10³ Pafor a PECVD process. Accordingly, the pressure within the corrective gap(31, 51 . . .) will typically be a low pressure. So, such a gaseousdielectric gap could be called as a “partial vacuum gap”.

[0071] In FIG. 6, the substrate 35′ (of a uniform thickness) is layingon a solid dielectric plate (surface 41 a) which can correspond to theceramic plate 41 of FIG. 4 in an inverted position. The front, innersurface 41 a of the plate 41 is flat, while its back surface 41 b isconvex and directly in contact with the lower metallic electrode 45, theinner surface of which is presently concave. So, the plate 41 is a sortof “lens”.

[0072] The electrodes 33, 45 illustrated in FIG. 7 correspond to theelectrodes of FIG. 5. The substrate 35′, which has a uniform thickness,is planar and parallel to the upper metallic electrode 33. Substrate 35′is laying on small posts 47 which are erected between the electrode 45and the substrate. The non planar internal upper surface 51 b of theelectrode 45 gives a non uniform thickness e₆ to the gaseous gap 61between the electrode 45 and the substrate 35′. Thus, the space 61 actsas a corrective dielectric layer for compensating the process nonuniformity and enables the substrate 35′ to be uniformly treated by theplasma discharge.

[0073] In FIG. 8, the two opposed electrodes 25, 33 have a uniformthickness, are planar and are parallel from each other. The tailoredlayer 71 is obtained from a non planar substrate 65 arranged on erectedposts 57. The elevations of such “spacing elements” 57 are calculatedfor giving the substrate 65 the required non planar profile.

[0074] The design of FIG. 8 should be mechanically the most attractive,because both electrodes 33, 25 remain flat and the profile of the smallgap 71 is defined by the inserts 57.

[0075] For any purpose it may serve, it will be noted that theradiofrequency power can be fed either on the electrode on which thesubstrate is attached, or on the opposite electrode.

[0076] In the examples of arrangements illustrated in FIGS. 1 to 8, itwill further be noted that the tailored layer (11, 31, 41, 51, 61, 71)will preferably have a thickness calculated as a Gaussian bell-shape forthe electrode to electrode distance (on the basis of the above-mentioned“central” arrangement). Then, said tailored layer itself will be deducedfrom a truncation of the bell-shape, what is left, namely the pedestalof the bell-shape after truncation is the space for the plasma gap(internal process space 13), and the substrate.

[0077] FIGS. 9 to 15 show other embodiments of an improved capacitivelycoupled radiofrequency plasma reactor, according to the invention.

[0078]FIG. 9 shows the most straightforward implementation of theinvention. The radiofrequency power source 9 is centrally connected toan upper electrode 3 called “shower head electrode” having holes 83through its lower surface facing the plasma process space 13, within theinner chamber 81 of the reactor 10. The counter-electrode 30 is definedby the metallic external wall of the chamber 81. The admission of thereactive gas is not illustrated. But the pumping of said reactive gas ismade through the exhaust duct 85.

[0079] It will be noted that all the mechanical (material) elementsarranged within the reactor 10 and illustrated in FIG. 9 are kept flat(electrodes and substrate 135, notably). However, the substrate 135(which has a uniform thickness e₇) is curved by laying it on series ofspacing elements 87 erected between the substrate and thecounter-electrode 30. The spacing supports 87 have variable height. Thesubstrate 135 is curved due to its own flexibility. The average distancebetween the supports is defined by the substrate thickness and its Youngmodulus.

[0080] In this arrangement, there are two layers in the space betweenthe electrodes that are not constant (uniform) in thickness: the plasmaprocess space 13 itself and the “corrective space” 89 behind thesubstrate. Although this example is not a straightforward solution, thisconfiguration is effective, because the RF power locally generated inthe plasma depends far more on the little variation of the thin“gaseous” capacitive layer behind the substrate, than the small relativevariation of the thickness e₈ of the plasma process space 13 (along thedirection of elongation of electrode 3).

[0081] The “corrective” tailored layer 89 is, in that case, behind thesubstrate. It is a gaseous (or partial vacuum) tailored layer, such awording “vacuum” or “gaseous” being just used to stress the fact thatthis layer has a dielectric constant of 1. The layer can contain gases(the dielectric constant is not affected).

[0082] There is a danger that the supports 87, whether they are metallicor dielectric, introduce a local perturbation of the process.

[0083] Indeed, just at the support level where the series capacitor ofthe tailored “corrective” layer 89 is not present, the RF field islocally going to be larger. The perturbation, as seen by the plasma, isgoing to spread over a given distance around the support. This distancescales as the substrate thickness e₇ plus the “plasma sheaththicknesses” (typically 2-4 mm) referenced as 13 a and 13 b in FIG. 9.

[0084]FIG. 9a shows a potential way to reduce to a bearable level theperturbation due to a support. The solution consists in surrounding eachspacing member 89 by a small recess 91. At the recess level, thecapacitive coupling is reduced. By adjusting the recess to make an exactcompensation, the local perturbation should be practically eliminated.

[0085] In relation to the invention, such an arrangement shows that thetailored “corrective” layer proposed in the invention should follow thetailored profile, on the average: very local perturbations on theprofile could be accepted as long as the capacitive coupling, remainssubstantially continuous and properly tailored, when averaged over ascale of a few millimiters.

[0086] In the arrangement of FIG. 9, the substrate 135 is a dielectricmember. This is important, since any tailored dielectric layer (such as89) must absolutely be within the space defined by the two extremelyopposed metallic layers defining the “process gap”. If a substrate ismetallic (electrically conductive), it screens off the effect of anyunderlying tailored capacity. Then, the substrate must be considered asone of the electrode.

[0087] In FIG. 10 is illustrated a rather common design in the processindustry. The reactor 20 is fed with two different driving energysources: a RF high frequency source (higher than 30 MHz) and a RF biassource 93 (lower than 15 MHz). The upper “shower head” electrode 3 isconnected to the high frequency source 91 and the low electrode 45 isconnected to the RF bias source 93.

[0088] One of the sources is meant to provide the plasma (in that case,we assume that it is an RF driving frequency with a rather highfrequency, through source 91). The other source 93 is presently used asan additive to provide an extra ion bombardment on the substrate 35.Typically, such an extra input (93) is plugged on the “susceptor” sideand is driven at 13.56 MHz.

[0089] Such a RF bias feature is often used in etching systems toprovide the reactive ion etching mode. It has been used in combinationwith many types of plasma (such as microwave, or electron cyclotronresonance).

[0090] In the example of FIG. 10, there are two electrodes (3, 45)facing each other. None of them is actually grounded. However, even inthat particular configuration, the tailored capacitor of the invention(layer 95 of a non uniform thickness) is appropriate. In the case ofFIG. 10, the configuration of FIG. 5 is implemented. An importantfeature is that the active part of the reactor 20 (plasma process space13, substrate 35, flat planar dielectric plate 21 of a uniform thicknessand tailored gaseous gap 95 of a non uniform thickness) is between twometallic plates (electrodes 3, 45). The fact that one is grounded or notand the fact that one or several RF frequencies are fed on one and/orthe other electrode, are irrelevant. The most important fact is thatthere is an RF voltage difference propagating between the two metallicplates 3, 45. In the example of FIG. 10, two RF frequencies are used.The drawing shows two injections (up and down) for the two RF waves. Itis arbitrary. They could be injected from the top together, or from thebottom (upper electrode 3 or lower electrode 45). What is important hereis that there are two different frequencies, one high frequency and onelow frequency. Both propagate in the capacitive reactor.

[0091] If, as proposed, a tailored capacitor such as 95 is introduced tocompensate for the high frequency non uniformity, it will make the “lowfrequency” non uniform. The “low” frequency wave amplitude will thenprovide a slightly hollow electric power profile due to the extratailored capacitor in the center. In other words, applying the“tailoring” concept of the invention here makes sense only if the “high”frequency local power uniformity is more important for the process thanthe “low” frequency power uniformity.

[0092] In FIG. 11, the tailored capacitive layer 105 is a gaseous spacebetween a ceramic liner 105 and the metallic electrode 109 which hasbeen machined to have the smooth and tailored recess (because of its nonplanar internal surface 109 a) facing the back part of the ceramic plate107. The ceramic liner 107 has many small holes 107 a which transmit thereactive gas provided by the holes 109 b in the backing metal electrode109. The reactive gas is injected through ducts 111 connected to anexternal gas source 113 (the pumping means are not illustrated). The RFsource 115 is connected to the electrode 109, as illustrated.

[0093] The design of the backing electrode 109 could have been atraditional “shower head” as electrode 3 in FIG. 10. Another option isthe cascaded gas manifold design which is shown in FIG. 11.

[0094] In FIG. 12, a microwave capacitive plasma reactor 40 isdiagrammatically illustrated. The illustration shows a possible designaccording to which a rather thick tailored layer generally referenced as120 (the thickness of which is designated as e₉) is used to compensatefor the drastic non uniformity due to electromagnetic propagation. Theillustrated reactor 40 is a reactor for etching a rather small wafer.The microwave comes from a coaxial wave guide 121 which expandsgradually at 122 (“trumpet” shaped) to avoid reflection. Then, themicrowave reaches the process zone 13 where the wave should converge tothe center of the reactor (which is cylindrical).

[0095] For the dimensions, the substrate 35 arranged on a flatcounter-electrode 126 has a diameter of about 10 cm, and an 1 GHz waveis generated by the microwave generator 123 (30 cm free space wavelength). The central thickness of the tailored layer 120 (if made ofquartz) should be about the same as the space 13 of the free plasmaitself.

[0096] It is presently proposed that the tailored layer 120 be obtainedfrom three dielectric plates defining three steps (discs 120 a, 120 b,120 c). The discontinuity of the steps should be averaged out by theplasma. The tailored layer is preferably very thick and it wouldactually make sense to call it “a lens”. The number of disks used toconstitute the lens could be four or higher if the ideally smooth shapeof the lens must be reproduced with a better approximation.

[0097] In said FIG. 12, it will be noted that the reactive gas isintroduced through the gas inlet 124, said reactive gas being pumped viaa series of slits (preferably radially oriented) through thecounter-electrode 126 and ending into a circular groove 125. The exhaustmeans for evacuating the reactive gas injected in the reactive spacebetween the electrodes are not illustrated.

[0098] In FIG. 13, the reactor 50 corresponds to the reactor 40 of FIG.12, except that, in this case, the step variation of the “corrective”dielectric layer 130 is not due to a change of thickness, but to achange of material constituting said layer 130 which has a uniformthickness along its surface. In other words, layer 130 is a variabledielectric constant layer having a uniform thickness e₁₀. The lowdielectric constant layer is the central plate 131 which isconcentrically surrounded by a second plate 132 having a mediumdielectric constant layer. The third external concentric plate 133 hasthe highest dielectric constant.

[0099] Hence, the equivalent thickest part of the tailored layer 130 ismade of the lowest dielectric material (quartz for example), whereas theintermediate layer 132 can be made of a material such as siliconnitride, the highest dielectric constant material at the periphery 133being presently made of aluminum oxide.

[0100] The example of FIG. 13 clearly shows that the dielectric layer ofthe invention having a capacitance per unit surface values which are notuniform along a general surface generally parallel to the substrate canbe obtained through a variation of the dielectric constant of saidlayer, while the thickness thereof remains uniform along its surface.

[0101] From the above description and the illustration of FIG. 14 (basedon the embodiment of FIG. 1), it must be clear that, in any case inwhich the thickness of the “corrective layer”, such as 140, is used tocompensate the process non-uniformity, as observed, the correctivelayer(s) will be the thickest in front of the location in the processspace (or on the facing electrode, such as 3) which is the farest awayfrom the electrode connection (9 a). It is to be noted that the “way”(referenced as 150) for calculating said “distance” must follow theexternal surface (such as 3 a) of the corresponding electrode.

[0102] Said thickness will be the lowest at the corresponding locationwhere the above “distance” is the smallest, and the non planar profileof the layer will follow said distance decreasing.

1. A capacitively coupled radiofrequency plasma reactor (1, 20)comprising: at least two electrically conductive electrodes (3, 5)spaced from each other, each electrode having an external surface (3 a,5 a), an internal process space (13) enclosed between the electrodes (3,5), gas providing means (7) for providing the internal process space(13) with a reactive gas, at least one radiofrequency generator (9)connected to at least one of the electrodes (3, 5), at a connectionlocation (9 a), for generating a plasma discharge in the process space(13), means (8) to evacuate the reactive gas from the reactor, at leastone substrate (15) defining one limit of the internal process space, tobe exposed to the processing action of the plasma discharge, said atleast one substrate (15) extending along a general surface (15 a) andbeing arranged between the electrodes (3, 5), characterized in that saidplasma reactor (1, 20) further comprises at least one dielectric layer(11) extending outside the internal process space, as a capacitorelectrically in series with said substrate (15) and the plasma, saiddielectric layer (11) having capacitance per unit surface values whichare not uniform along at least one direction of said general surface (15a), for generating a given distribution profile, especially forcompensating a process non uniformity in the reactor.
 2. A capacitivelycoupled radiofrequency plasma reactor comprising: at least twoelectrically conductive electrodes (3, 45) spaced from each other, eachelectrode having an external surface (3 a, 5 a), an internal processspace (13) enclosed between the electrodes (3, 5), gas providing means(7) for providing the internal process space with a reactive gas, aradiofrequency generator (9, 91) for geneating a plasma discharge in theprocess space (13), said generator connected to at least one of theelectrodes (3, 45) at a connection location, preferably centrallyarranged on said electrodes, an additional radiofrequency generator (93)connected to at least one of the electrodes (3, 45), for increasing theion bombardment on said substrate, means (8) to evacuate the reactivegas from the reactor, the at least one substrate (35) defining one limitof the internal process space to be exposed to the processing action ofthe plasma discharge, said at least one substrate extending along ageneral surface and being arranged between the electrodes, characterizedin that said plasma reactor (1, 20) further comprises at least onedielectric layer (95) extending outside the internal process space, as acapacitor electrically in series with said substrate (35) and theplasma, said dielectric layer (11) having capacitance per unit surfacevalues which are not uniform along at least one direction of saidgeneral surface (15 a), for generating a given distribution profile,especially for compensating a process non uniformity in the reactor. 3.The reactor of claim 1 or claim 2 , characterized in that saiddielectric layer has a thickness (e₁) along a direction perpendicular tothe general surface of the substrate, said thickness being non uniformalong said dielectric layer, so that the reactor has said locationdependent capacitance per unit surface values.
 4. The reactor accordingto claim 3 , characterized in that: the said dielectric layer (15) isthe thickest in front of the location in the process space (13) which isthe farest away from said connection location (9 a) where theradioirequency generator is connected to said at least one electrode,and said thickness decreases from said process space location as thedistance between the process space location and the connection locationon the corresponding electrode decreases.
 5. The reactor according toanyone of claims 1 to 4 , characterized in that said dielectric layer(15) has at least one non planar-shaped external surface.
 6. The reactoraccording to anyone of claims 1 to 5 , characterized in that at leastone of said electrodes has a non planar-shaped surface facing thesubstrate.
 7. The reactor of anyone of claims 1 to 6 , characterized inthat: said one dielectric layer is locally delimited by a surface of oneof said electrodes (5 a, 41 b, 51 b), and said delimitation surface ofsaid one electrode is curved.
 8. The reactor according to anyone ofclaims 1 to 7 , characterized in that said dielectric layer comprises atleast one of a solid dielectric layer and a gaseous dielectric layer, ora combination of the both mentioned.
 9. The reactor according to anyoneof the preceding claims, characterized in that the at least onesubstrate comprises a plate having a non planar-shaped external surface.10. The reactor of anyone of the preceding claims, characterized in thatthe at least one substrate (65) has a curved shape.
 11. The reactoraccording to anyone of the preceding claims, characterized in thatspacing members are arranged between said substrate (35′, 65) and one ofthe electrodes (25, 45), said spacing members having elongations beingnon uniform.
 12. The reactor according to claim 11 , characterized inthat the spacing members (89) at the non-substrate-end being surroundedby a space (91), for at least partially compensating the electromagneticperturbation induced by the contact between the spacing member and thesubstrate.
 13. A process for treating at least one substrate (15, 35′,65) in a radiofrequency plasma reactor (1, 20), comprising the steps of: locating the at least one substrate (15, 65) between two electrodes(3, 5), the at least one substrate extending along a general surface (15a), having a circulation of a reactive gas within the reactor, so thatsuch a gas is present in an internal process space (13) arranged betweenthe electrodes, having a radiofrequency generator (9) connected to atleast one of the electrodes (3, 5), at a connection location (9 a),having a plasma discharge in at least a zone of the internal processspace (13) in such a way that said substrate is exposed to theprocessing action of the plasma discharge, characterized in that itfurther comprises the steps of creating an extra-capacitor electricallyin series with said substrate and the plasma, said extra-capacitorhaving a profile, and defining the profile of the extra-capacitor insuch a way that it has location dependent capacitance per unit surfacevalues along at least one direction of the general surface of thesubstrate, for generating a given distribution profile, especially forcompensating a process non uniformity in the reactor.
 14. The processaccording to claim 13 , characterized in that the radiofrequencydischarge is generated at a frequency higher than for example 1 MHz,preferably higher than 19 MHz, the at least one substrate has a surfacelarger than 0.5 m², and the largest dimension of the substrate surfaceexposed to the plasma discharge is higher than 0.7 m.
 15. The process ofclaim 13 or claim 14 , characterized in that the step of defining theprofile of the extra-capacitor comprises the step of defining such aprofile having a non planar-shape along a surface, in such a way thatsaid extra-capacitor is materially defined by at least one dielectriclayer having a non uniform thickness along said surface.